Where do the Prime Meridian and the Equator meet? The Royal Observatory lies at Longitude 0° by the original definition of Airy's Transit Circle, and at Latitude is 51° 28' 38'' N. Latitude and longitude are divided into degrees (°), minutes (′) and seconds (″), with sixty minutes in a degree and sixty seconds in a minute. Where longitude is the distance east or west of the Prime Meridian line, latitude is measured by the distance north or south of the equator. What is the latitude of the Royal Observatory? The cross-hairs in the eyepiece of the Transit Circle precisely defined Longitude 0° for the world.įind out more about Airy's Transit CircleĪs the Earth’s crust is moving very slightly all the time the exact position of the Prime Meridian is now moving very slightly too, but the original reference for the prime meridian of the world remains the Airy Transit Circle in the Royal Observatory, even if the exact location of the line may move to either side of Airy’s meridian. The transit circle was built by the 7th Astronomer Royal, Sir George Biddell Airy, in 1850. In 1884 the Prime Meridian was defined by the position of the large 'Transit Circle' telescope in the Observatory’s Meridian Observatory. Greenwich was chosen as the centre for world time.įind out more about Greenwich Mean Time Where is the Prime Meridian? When the railway and communications networks expanded in the 1850s and 1860s, there needed to be an international time standard. There were no national or international conventions which set how time should be measured, or when the day would begin and end, or what length an hour might be. Since the late 19th century, the Prime Meridian at Greenwich has served as the reference line for Greenwich Mean Time, or GMT.īefore this, almost every town in the world kept its own local time. The line itself divided the eastern and western hemispheres of the Earth - just as the Equator divides the northern and southern hemispheres. If you stand with one foot on one side and the other on the left, you are perfectly in the middle of east and west, according to the prime meridian line. Every place on Earth was measured in terms of its distance east or west from this line. The line in Greenwich represents the historic Prime Meridian of the World - Longitude 0º. The decision was based on the argument that by naming Greenwich as Longitude 0º, it would be advantageous to the largest number of people. Therefore the Prime Meridian at Greenwich became the centre of world time. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian. The first was the fact that the USA had already chosen Greenwich as the basis for its own national time zone system. There were two main reasons for the choice. Why does the Prime Meridian run through Greenwich? By comparing thousands of observations taken from the same meridian it's possible to build up an accurate map of the sky. A meridian is a north-south line, selected as the zero reference line for astronomical observations.

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